In recent years, much effort has been made to develop two-dimensional X-ray sensors (also referred to as “FPDs (Flat Panel Detectors)) for radiography in order to acquire digital data for large screens (see Patent Document 1: Japanese Patent Laid-Open No. 09-288184). A radiographic device has been applied to medical radiography; the radiographic device performs a plain radiographic operation using a two-dimensional X-ray sensor having a large detector surface of size 43×43 cm.
On the other hand, another radiographic device is known which exposes the object to X rays and uses an X-ray detector to detect the dose of X-ray radiation having penetrated the object, thus obtaining projection image data (sometimes called “scanogram” or “SCOUT image”), tomogram, or three-dimensional image data on the object on the basis of the X-ray detection output (photon count of the X ray) from the X-ray detector.
An improvement in a technique for developing a two-dimensional X-ray sensor has resulted in development of a cone beam CT (Computed Tomography) device (hereinafter referred to as a CBCT device) that is a radiographic device that generates three-dimensional image data. Conventional X-ray CT devices use X-ray beams that are sliced thin in a Z direction (body axis direction); these X-ray beams are called fan beams. In contrast, the CBCT device uses X-ray beams spreading also in the Z direction (hereinafter referred to as “cone beams”) and receives the cone beams using a two-dimensional detector (having a built-in two-dimensional X-ray sensor). Compared to the CT device using fan beams, the CT device using cone beams can image a wider range of the object during one rotation of scan. This reduces the number of rotations required for computed tomography, thus advantageously allowing a radiographic operation to be efficiently performed. In other words, a radiographic operation can be efficiently performed by increasing a cone angle (spread angle in the Z direction) corresponding to the spread of cone beams. On the other hand, too large a cone angle disadvantageously results in an error in image generation.
A certain type of cone beam CT device (hereinafter referred to as a CBCT device) carries out scanning (collects image data) while rotating a pair of an X-ray source and a detector around the object as shown in Patent Document 2 (Japanese Patent Laid-Open No. 10-21372) (image data may be referred to as “projection images”). However, to irradiate the entirety of the large detector surface of a two-dimensional detector with radiation from a radiation source, having a cone angle less than a given angle, at least a given distance must be set between the two-dimensional detector and the radiation source. Accordingly, with a CBCT device of a type that carries out scanning (collects image data) while rotating a pair of an X-ray source (X-ray generation device) and a detector around the object, it is very difficult, owing to the size of the device, to set a certain distance between the two-dimensional detector and the radiation source so as to allow the large detector surface of the two-dimensional detector to be effectively used.
On the other hand, effort has been made to develop and put to practical use an object rotating type CBCT device that rotates the object while a pair of an X-ray source and a two-dimensional detector has been fixed as described in Patent Document 3 (Japanese Patent Laid-Open No. 2000-217810). The object rotating type CBCT device acquires image data at every given rotation angle and uses a plurality of acquired image data to reconstruct an image.
The penetration rate of X rays varies depending on the direction in which the X rays are incident on the human body. Further, to obtain a predetermined image SN ratio, it is necessary that X-ray radiation of at least a given dose reaches the two-dimensional detector. Moreover, the dose of X-ray radiation to which the patient is exposed needs to be minimized. Thus, the dose of X-ray radiation reaching the two-dimensional detector during acquisition of one image data desirably falls within a predetermined range.
The following is used to solve the above problems, in a spiral CT and a multi-slice CT which are the traditional type of X-ray CT device rotating the X-ray source: (1) a method of adjusting X rays emanated by the X-ray generating means, for each slice position on the basis of projection image data or (2) a method of adjusting X rays emanated by the X-ray source, at every rotation angle on the basis of data on the dose of X-ray radiation having penetrated the patient. In other words, the conventional CT devices adjust the dose of X-ray radiation reaching the detector (sensor unit) by adjusting the output from the X-ray generation device (Patent Document 4 (Japanese Patent Laid-Open No. 07-124152) and Patent Document 5 (Japanese Patent Laid-Open No. 08-206107)).
On the other hand, the object rotating type CBCT device can be used as a radiographic device for normal plain radiography by stopping the rotation of the object (testing object). That is to say, the single device can advantageously be used both for CT and for plain radiography. Further, it has been proposed that X-ray generation devices for plain radiography already installed in general hospitals be diverted to object rotating type radiographic devices. Moreover, it has been desired that a single X-ray generation device be used both as a radiographic device for plain radiography and as an object rotating type radiographic device.